The present invention relates generally to gas springs and, particularly, to gas springs of the type that are used primarily to lift and hold open the trunk lids, tailgates, hatchback lids, and engine compartment hoods of motor vehicles.
Gas springs are widely used to partly or totally counterbalance the engine compartment hoods, trunk lids, rear windows and tailgates of passenger cars, station wagons, and vans to facilitate opening them and to hold them open at a nearly or fully open position. It is well-known that the force outputs of gas springs vary considerably with the temperature of the gasxe2x80x94at low temperatures the gas spring produces a force that can be very much lower than the force produced at high temperatures. It is necessary, therefore, to design a gas spring so that it produces a sufficient force to hold open the hood, tailgate or the like (hereinafter referred to as the xe2x80x9cloadxe2x80x9d) at a suitably selected low temperature, say, xe2x88x9230 degrees F. Ordinarily, gas springs are designed to provide a force of from about one to about five pounds over the load in the hold-open position of the load at the low temperature. At high temperatures, the hold-open force may increase by as much as 50 pounds, which means that the force required to move the load toward closed from the hold-open position (the xe2x80x9chandle loadxe2x80x9d) can be more than 50 pounds.
In addition to the problem of wide variations in the handle load as a function of temperature, the counterbalancing force exerted by the gas spring on the load at all positions of the load between closed and open varies widely with temperature. In cold weather, the gas spring force exerts a considerably lower counterbalancing force on the load than at high temperatures. Depending on the geometry of the gas spring/load system, the user may have to exert a relatively large force on the load during part or all of the movement of the load from closed to fully open in cold weather. In hot weather the gas spring force may move the load from closed to open without the intervention of the user under a relatively high opening force and at a relatively high speed, which can sometimes be disconcerting to an unwary user or can damage the load if there is an obstruction that prevents the load from fully opening.
Various proposals have been made for mitigating the problem of variations due to temperature changes in the hold-open/handle load with the load open. Some examples are found in U.S. Pat. No. 5,106,065 (Staton et al., 1992) and U.S. Pat. No. 5,404,972 (Popjoy et al., 1995). Those solutions are based on having one or more bypasses between the sub-chambers of the gas chamber on opposite sides of the piston, each bypass having a spring-biased valve which is closed at low temperatures and blocks the flow of gas from the closed-end sub-chamber to the rod-end sub-chamber and opens at high temperatures to permit gas to flow through the bypass. When the bypass is closed, the relatively low force of the gas spring is supplemented by the effect of the biasing force on the valve such as to increase the hold-open force. The devices of those two patents have no effect on the force output of the gas spring when the load is moved from closed to open.
It has also been suggested that a reduction in the output force of a gas spring due to leakage of gas or low temperature can be compensated for by providing a secondary gas chamber or reservoir that contains gas at a pressure higher than that in the primary chamber and a pressure-responsive valve in a passage that connects the secondary chamber to the primary chamber and opens when the pressure in the primary chamber falls below a predetermined value. Such an arrangement is proposed in U.S. Pat. No. 5,042,782 (Mitgen, 1991). When gas is supplied from the secondary chamber to the main chamber to make up for a pressure drop in the main chamber due to a decrease in temperature, the output force is prevented from being reduced by the addition of air to the main chamber from the secondary chamber. When the force output of the gas spring rises due to an increase in temperature, gas can, if desired, be released from the main chamber through a vent (FIG. 4), thereby preventing the gas spring force from increasing to an undesirable level. The arrangement of the Mintgen patent maintains a relatively uniform force output of the gas spring over a wide range of temperatures but inherently requires releasing gas from the system, lest complete hot-cold-hot cycles of operation cause the operating force to become excessive on the next cold-hot phase of another cycle. Unless a wide difference is built into the respective valves for the replenishment of gas and release of gas, frequent venting of gas will soon deplete the gas in the secondary chamber, thus requiring recharging. The need for frequent service of the system is a disadvantage.
An object of the present invention is to provide a gas spring that provides a controllable force output over a long useful life without the need for any service, such as replenishing a gas in a supplemental supply reservoir. Another object is to provide a gas spring that is capable of exerting forces that vary in a predetermined manner during each stroke of an operating cycle. Yet another object is to provide a gas spring that provides a force output that does not vary with temperature changes.
The objects referred to above are attained, in accordance with the present invention, by a gas spring for moving a load relative to a body that includes a cylinder member having an axis, an inner surface forming a chamber, a rod end and a closed end, a piston rod seal/guide received in the rod end of the cylinder member, and a piston rod received in the seal/guide for movement into and out of the cylinder member. A floating piston is received in the cylinder member for movement along the axis and in sealed engagement with the inner wall to define in the chamber a gas section between the floating piston and the rod end and a closed end section between the floating piston and the closed end. A mass of gas under a pressure above atmospheric pressure is contained in the gas section. A powered drive is provided for moving the floating piston axially of the cylinder member to vary the volume of the gas section of the chamber. A sensing device senses a characteristic of the gas spring that is indicative of the force applied to the rod by the gas in the gas section and produces a signal indicative thereof. The signal generated by the sensing device is used to control the drive.
A gas spring, according to the present invention, provides a predetermined force output by variation of the volume of the gas section. For example, if the temperature of the gas in the gas section drops but the volume of the gas section remains constant, the pressure of the gas and the force output of the gas spring are reduced correspondingly. The reduction in pressure is detected by the sensor. The controller responds to the change in the sensor signal by activating the power drive, which displaces the floating piston in a direction to reduce the volume of the gas section, thereby raising the pressure of the gas and the output force. The present invention makes it possible for the gas spring to provide an output force that remains substantially constant for any given stroke position, regardless of variations in temperature over a very wide range.
In a simple system the controller and drive may be activated periodically when the load is closed. The controller and drive are powered by the battery of the vehicle. Periodic activation saves battery power. A low-battery shut-off can be incorporated to prevent the battery from being drained. If the sensor detects a change in the gas pressure, the drive is activated to change the volume of the gas section and restore the pressure to the predetermined level. Therefore, when a user opens the load, the gas spring has been previously adjusted to provide the desired output force to counterbalance the load, maintain the load in the open position, and yield to a predetermined handle load in the closing direction.
In one form of system, the drive is deactivated when the load is moved from closedxe2x80x94e.g., by responding to an existing lamp switch. After opening is initiated, the gas spring functions in the same way as a conventional gas spring without intervention of the drive.
As the rod of a gas spring extends, the volume of the part of the rod that was previously within the gas chamber becomes part of the volume occupied by the gas. As is well-known, of course, that phenomenon reduces the output force as a function of rod extension. It may be desirable, therefore, to have the drive activated only during the opening stroke or when the load is open so that a predetermined force is established at the open position of the load.
Opening of the load may also initiate a phase of operation of the system in which the floating piston is moved in response to a computer program stored in the controller that provides predetermined variations in the gas spring force by driving the floating piston in response to signals generated under the control of the computer program. For example, the program may provide for automatic opening of the load without the intervention of the user, an idle or dwell state when the load is fully opened or opened to a desired degree, maintenance of a predetermined hold-open force, and response to a predetermined handle load that initiates automatic closing of the load. Ancillary to computer control of the gas spring at the beginning of an opening stroke is the possibility of providing a relatively high output force at the beginning of the opening stroke, when the mechanical advantage of the gas spring relative to the load is usually low due to the geometry of the gas spring relative to the load, followed by a reduction in force at the end of the opening stroke. The sensor signal provides feedback for servo-control of the drive device to match the function of the gas spring to the program.
The drive device for moving the floating piston may include a mass of oil in the closed section of chamber, a pump for selectively adding oil to the closed section, an oil sump serving the oil pump, a power source for energizing the pump, and conduits and a controllable valve interposed between the closed section of the chamber and the pump and the sump. With such a hydraulic drive, the characteristic indicative of the gas pressure sensed by the sensing means is, preferably, the pressure of the oil in the closed section of the chamber. Advantageously, an emergency valve can be interposed between the closed section and the oil delivery/return conduit and adapted to close in response to a flow of oil from the closed section to the conduit at a rate exceeding a predetermined rate. Such an arrangement prevents the load from closing abruptly in the event that there is a failure in the hydraulic system.
In other embodiments, the device for moving the floating piston includes an electric motor and a transmission, such as a ball-screw drive, coupled between the motor and the floating piston. In such embodiments, the characteristic sensed by the sensor may be the pressure of the gas in the gas section of the chamber. The sensor is located in the floating piston and the pressure signal is taken out of the cylinder through a conductor located within the closed section of the cylinder chamber and passing through the end closure unit.